1. Field of the Invention:
The present invention relates to a device for locking/unlocking rotation of an eccentric bearing interposed between a piston-pin and a connecting rod of an internal combustion engine to change a compression ratio of the engine according to the rotation of the eccentric bearing.
2. Description of the Prior Art:
In an Otto-cycle internal combustion engine it is desirable to increase the compression ratio, because an increased compression ratio brings about an improved fuel efficiency as well as an increased axial torque. However, the increase of the compression ratio will be limited, because increase in the compression ratio is accompanied with an increased probability of causing knocking in the engine when the gas in the combustion chamber is adiabatically compressed and the temperature of the gas rises. The knocking is liable to occur under high engine loads when an amount of air introduced into the combustion chamber is large and is less liable to occur under low engine loads when an amount of air introduced into the combustion chamber is small. Therefore, it is desirable to make the compression ratio changeable according to engine loads so that the compression ratio may be high under medium and low engine loads and may be low under high engine loads.
To satisfy these requirements various proposals have been made concerning compression ratio changing devices for an internal combustion engine. Japanese Utility Model Publication SHO No. 58-137832 discloses the eccentric bearing-type device in which the eccentric bearing having a cylindrical inside surface eccentric with respect to a cylindrical outside surface is installed between the piston-pin and the connecting rod. When the connecting rod rotates, the relative position of the piston with respect to the connecting rod is changed and the compression ratio of the engine is varied.
To explain the problems in the conventional eccentric bearing-type device, the structure of the conventional device will be explained while referring to FIGS. 26 to 31. Eccentric bearing 1 having inside surface 2 and outside surface 3 is rotatably interposed between the piston-pin and the surface of the small end hole of connecting rod 5. Lock hole 4 is formed in eccentric bearing 1 and lock-pin hole 6 is formed in connecting rod 5. The centers of lock hole 4 and lock-pin hole 6 are included in a common plane which is at a right angle with respect to the axis of eccentric bearing 1. Lock-pin 7 is slidably inserted into lock-pin hole 6 to be able to move into lock hole 4. When lock-pin 7 in lock-pin hole 6 moves toward lock hole 4 and one portion of lock-pin 7 comes into engagement with lock hole 4, the rotation of eccentric bearing 1 is locked and when lock-pin 7 moves in the direction apart from lock hole 4 and disengages with lock hole 4, the rotation of eccentric bearing 1 becomes free.
When the rotation of eccentric bearing 1 is locked, the compression ratio is fixed. Since lock hole 4 is provided at such a position that lock hole 4 receives lock-pin 7 where the thickest wall portion of eccentric bearing 1 comes to the lowest position with respect to the axis of eccentric bearing 1, the fixed compression ratio is high. When the rotation of eccentric bearing 1 is unlocked, eccentric bearing 1 rotates around its axis receiving the moment which is produced by the loads on the piston and the arm of eccentricity of eccentric bearing 1. The loads on the piston include the combustion pressure, the compression force of the gas and the inertia force of the piston. When the piston is at T.D.C. of the compression stroke, eccentric bearing 1 rotates and naturally takes the rotational position in which the thickest wall portion of eccentric bearing 1 comes to the highest position with respect to the axis of eccentric bearing 1 and the compression ratio becomes low. In this way the compression ratio is changed by locking and unlocking the rotation of eccentric bearing 1 by means of lock-pin 7.
To make the engaging of lock-pin 7 with lock hole 4 smooth, guide groove 8 is formed in the radially outer portion of eccentric bearing 1. Guide groove 8 extends circumferentially and the center of guide groove 8 is in the plane which includes the center of lock hole 4. Guide groove 8 begins at an outside surface of eccentric bearing 1 apart from lock hole 4 and becomes gradually deep toward lock hole 4 where guide groove 8 terminates. The portion of the surface of lock hole 4 opposing guide groove 8 constitutes colliding surface 9 which collides with lock-pin 7 and stops the rotation of eccentric bearing 1, thereby allowing lock-pin 7 to enter lock hole 4 smoothly.
However, the smooth entry of lock-pin 7 into lock hole 4 can be obtained only when eccentric bearing 1 rotates in the normal direction of arrow N shown in FIGS. 26 to 28, because lock-pin 7 collides with colliding surface 9. When eccentric bearing 1 rotates in the reverse direction of arrow R, shown in FIGS. 29 to 31, lock-pin 7 can not collide with colliding surface 9 and may jump lock hole 4. If such jumping occurs, eccentric bearing 1 can not be locked by lock-pin 7 and, as a result, high compression ratio states can not be produced.